1. Field of the Invention
The present invention relates to an image display device capable of displaying an image with light from the front, light from the back or both the light from the front and the light from the back, and an image sheet.
2. Description of the Related Art
Conventionally, an image display device for ornament or for advertisement is often installed in a station yard, an interior wall of a hotel or a department store, an exterior wall of a building or the like.
Generally, such an image display device has a structure where an image sheet on which an image is formed is mounted in an appropriate panel frame and a fluorescent lamp is installed as a backlight so as to illuminate the image sheet from a back surface thereof. An identical image is printed on both surfaces of a support including translucent milky white films, which is used as the image sheet. Instead of printing, photographic printing is performed for forming an image sheet (Japanese Unexamined Patent Publication No. 59-222835).
In such an image display device, an image can be seen clearly at appropriate density with transmitted light by illumination from the back. Even when the illumination from the back is turned off, to some extent, the image can be clearly seen at appropriate density with reflected light of external light.
In the conventional image display device mentioned above, however, a fluorescent lamp is used as a backlight. Accordingly, a lighting system is large in size and shape, making the entire image display device large in size and weight.
It is conceivable that an EL (electroluminescence light) is used as a lighting system for such a backlight.
Zinc sulfide is usually used as a light emission source of the EL for enhancement of luminance. In this case, since a light emission color becomes blue green, the EL cannot be used as a backlight for a color image for practical purposes. Then, a correction is so performed that white light or a daylight color is emitted by coating a surface of the light emission source with pink (peach color/red species) that is a color opposite to blue green. However, in the case of representing a color image using the light source whose surface color is pink, when an image is printed on a milky white sheet-like material and is used, a problem arises in which exact colors cannot be displayed.
More specifically, in the case of seeing an image formed on a milky white sheet-like material by using the EL as a backlight, the surface color of pink influences a color of the image when the image is seen by any of reflected light (front light) or transmitted light (backlight), even if the usage differs to some extent in light intensity between the reflected light and the transmitted light. As a result, the color of the image changes to a pinkish color and exact colors cannot be represented.